The central hypothesis of this proposal is that successful reconstitution of humoral immunity following bone marrow transplantation is dependent upon the recapitulation of two distinct ontogenic programs. The first initiates during embryogenesis and results in the production of progenitors destined to generate B-1 B cells that are particularly efficient at recognizing encapsulated bacteria such as S. pneumoniae that express carbohydrate antigens. The predominant B cell population, referred to as B-2 B cells, is produced in a second wave of lymphopoiesis that initiates just before birth and is sustained throughout adult life. In comparison to B-2 development, our understanding of fetal B cell development and B-1 B lymphopoiesis is incomplete. Aim 1 will identify the fetal tissues with which B-1 B cell developmental potential is associated. Particular emphasis will be placed on when and where a recently identified CD45R-/low CD19+ B-1 B cell specified progenitor first develops. While these initial studies will be performed in mice, studies to characterize novel human cord blood B lineage populations that include an intermediate that co-expresess B and myeloid lineage determinants will also be performed. Numerous questions exist regarding the development of mature IgM expressing B-1 B cells. For example, few B-1 B cells are present at birth, and questions remain about where B-1 B cell progenitors are found and the tissues that support their maturation. Finally, the spleen has been implicated in the generation and/or maintenance of B-1 B cells, but its precise role has not been elucidated. Aim 2 will address these issues. Bone marrow from young adult mice can reconstitute B-1 B cells, but it is unknown if this potential is maintained throughout life. Whether the B-1 cells that are generated from postnatal bone marrow derive from adult hematopoietic stem cells or fetal derived B-1 progenitors that survive into postnatal life is also not known. These issues, which are of relevance when considering the age of marrow transplant donors, will be the focus of Aim 3. The information obtained from these studies will provide new information regarding fetal and adult B cell development and the development of the humoral immune resonse. This information in turn is relevant to restoration of humoral immunity following bone marrow transplantation, the treatment of immunodeficiency diseases, and the recovery of the immune response following various myeloablative treatments.